For production of hydrogen as an energy source for the next generation are known, for example, a process by electrolysis of water, and a process for obtaining hydrogen by steam reforming of various raw gases such as methanol, propane gas, liquefied natural gas and city gas. In particular, in the latter process, a mixed gas containing hydrogen gas is obtained by reforming and conversion of these gases. However, for utilizing the hydrogen gas as a fuel to generate electricity, it is required to separate a hydrogen gas having a high purity of 99.99% or more.
When a natural gas is used as a raw gas, the following hydrogen separation process is used as shown in FIG. 8. After desulfurizing in a desulfurizer “a” at 350° C., the natural gas is passed through a reformer “b” at 800° C. to which steam for reforming is introduced, a high temperature CO shift converter “c” at 400° C., and a low temperature CO shift converter “d” at 250° C. to produce hydrogen, and the hydrogen is taken out of a PSA unit “e” (hydrogen purification apparatus by catalyst adsorption) at 100° C. or less.
However, in this process using the PSA, the equilibrium reaction temperature is as high as about 800° C. Further, in addition to complexity and size increase of plant itself and increase in process steps and number of equipments, the plant cost is high and the maintenance of plant is troublesome. Moreover, the purity of the obtained hydrogen gas is not satisfactory and, therefore, improvement of this process is desired also from the viewpoint of hydrogen gas purification efficiency, so the process has not sufficiently spread.
As a process capable of solving these problems is recently known a process in which steam reforming of a raw gas is carried out in a downstream of the desulfurizer “a” as shown in FIG. 9. After this reforming, a high purity hydrogen gas is obtained in a membrane reactor “f” using a hydrogen separation membrane. According to this system, a low temperature of, for example, about 550° C. is sufficient for reforming since catalysts are used and a non-equilibrium reaction occurs. For example, in case of using natural gas as a raw gas, the reforming proceeds according to a reaction of CH4+2H2O→4H2+CO2, and hydrogen and off-gas (carbon dioxide) are separated and taken out.
Like this, hydrogen can be purified and separated through two steps from an introduced raw gas and steam, and the off-gas is also taken out and reused to utilize its temperature. In this process, the hydrogen separation unit using the membrane reactor “f” can be operated at a relatively low temperature. Thus, an apparatus for this process can be greatly downsized and simplified as compared with apparatuses for a conventional process, and it can be utilized as an on-site apparatus such as an apparatus for use at gas stations. It is also expected to utilize it as a high purity hydrogen generator for fuel cells.
A hydrogen separation element in such an apparatus is disposed so that a hydrogen-permeable membrane in the form of a thin film or membrane made of Pd which is known as a metal selectively permeable to hydrogen, or its alloy is located on the raw gas side. This hydrogen-permeable membrane is usually supported by a porous support. Hydrogen gas separated by the hydrogen-permeable membrane is taken out outwardly through the support. Therefore, the support withstands a pressure of fed gas applied to the hydrogen-permeable membrane and prevents the membrane from deforming. It is also used as a flow passage member for well flowing down the separated hydrogen gas.
As to such hydrogen separation members, a hydrogen-permeable membrane is directly formed on a porous support by a method such as electrolytic plating, electroless plating or chemical vapor deposition method. It is known, for example, from Patent Literature 1 that according to such a method, the hydrogen separation efficiency per unit volume (volumetric efficiency) can be increased. It is also disclosed, for example, in Patent Literature 2 that a gas separation membrane having a surface area increased to improve the treatment amount is obtained by forming an inorganic porous or organic membrane support having a large number of concave and convex portions by using a mold prepared by electroforming, and then applying electroless plating to the surface of the membrane support to form a separation membrane thereon.
Further, in Patent Literature 3 is proposed a separator having an improved durability to high temperature and high pressure together with a high flux capacity and a low cost performance, which is prepared by using a mesh screen as a supporting structure, superposing a hydrogen separation foil on the screen, pressing it by a roller or the like to follow the undulated concave-convex surface of the mesh, and joining them at the contact portions thereof.
Patent Literature 1: JP-A-2002-239353
Patent Literature 2: JP-A-2001-29761
Patent Literature 3: JP-A-2001-162144